Heretofore, known as image forming apparatuses such as electro-photographic copying machines, printers, facsimiles, and the like, to transfer a toner image on an electro-photographic photoconductor (hereinafter also referred simply to as a photoconductor) onto a recording medium to form a final image, has been a system which transfers a toner image, formed on a photoconductor, directly onto a recording medium. Further, known has been an image forming system employing an intermediate transfer body. In the latter system, another transfer process is included which transfers a toner image from the photoconductor onto a recording medium, and after performing the primary transfer from photoconductor onto the intermediate transfer body, the primary transfer image on the intermediate transfer body is subjected to secondary transfer onto another recording medium, whereby a final image is produced. In the place where secondary transfer onto a recording medium is carried out, there is often provided a secondary transfer roller or secondary transfer belt which faces the intermediate transfer body and forms a nip therebetween. The method using an intermediate transfer device is often utilized as a superimpose transfer method of each color toner image in so-called a full-color image forming apparatus in which an original image which has been separated into colors is reproduced employing the color subtractive process using color toners such as yellow (Y) toner, magenta (M) toner, cyan (C) toner, and black (K) toner.
According to such an image forming apparatus, a peculiar problem has been known in which the rotation speed of photoconductor or intermediate transfer body is varied at the moment when a recording medium is fed into, or fed out from, the transfer section where transfer of a toner image from photoconductor or intermediate transfer body onto a recording medium is carried out, the recording medium causes an instantaneous impact onto the photoconductor and intermediate transfer body resulting in fluctuations of load torque.
This problem, mentioned above, often introduces the misalignment of transferred image in the primary transfer and an uneven exposure onto photoconductor during exposure process, resulting in the deterioration of image quality.
Accordingly, techniques to solve the above-mentioned problem have been proposed by the following patent documents.
For example, Japanese Patent Application No. 2007-316427 discloses an embodiment in which such adverse effect, caused by the impact by recording medium to transfer section, is prevented by controlling the pressure to be applied to the secondary transfer roller, which forms a nip in the transfer section, in accordance with the timing of feed-in and feed-out of the recoding material, by using a piston cylinder encapsulating a fluid of which viscosity changes according to the applied electrical field, or magnetic field.
Also, Japanese Patent Application No. 2007-286382 discloses an embodiment in which drastic decline of the rotation speed of intermediate transfer body driving motor is prevented by control to reduce the pressure to be applied to the secondary transfer roller, by providing a pressure adjusting means which adjusts pressure to be applied to the secondary transfer roller which forms a nip for secondary transfer by being abutted to the intermediate transfer body, at the moment when a recording medium is fed into the secondary transfer nip.
According to the above mentioned patent documents, however, it is necessary to provide a mechanical structure and an electrical control for exclusive use. Also, it is often difficult to detect the exact timing of when a recording medium is fed into the transfer section and fed out, and to control the transfer pressure to increase or decrease at the moment when the recording medium is fed in or fed out, in response to a wide variety of recording mediums.
Furthermore, in a case in which the entire secondary transfer section, which consists of a secondary transfer belt, a driving motor to drive the secondary transfer belt, a driven roller, a tension roller, a cleaning section to clean off residual toner, and the like, is driven in order to form a secondary transfer nip, the weight of the entire secondary transfer section increases, and therefore, resulting in problem in that the secondary transfer section vibrates widely when a recording medium is fed in, and also, it takes longer time for the vibration to settle down.